One of the means by which operating control of the fissioning process in neutronic reactors is achieved, is through the limited absorption of neutrons released therein. The fissioning reaction within a nuclear core can be controlled by selective addition and removal of neutron absorbing material, typically in the form of absorber rods. Generally, a plurality of channels is provided in the core of a reactor, with the channels being adapted to receive rod-like control members having a combination of fuel element material and neutron-absorbing material. Such members are selectively moved to positions within the core of the reactor, so as to limit the neutron chain reaction therein such that a predetermined power level is maintained. The members containing neutron-absorbing material are generally actuated by electro-mechanical devices mounted on the reactor and operated remotely therefrom.
Neutronic reactors are also provided with shutdown means to suddenly terminate or quench the chain reaction taking place therein. Such means are provided for reasons of safety; that is, in the event that the ordinary control means fail, it is essential that the chain reaction be prevented from exceeding the physical limitations of the neutronic reactor. One means of terminating the chain reaction is to suddenly introduce safety members of neutron-absorbant material into the core of the neutronic reactor. The safety members are typically inserted into the core of a neutronic reactor by mechanisms similar to those utilized for the movement of control members. It is with such safety members and reactor shutdown assemblies that the present invention is concerned.
In a nuclear reactor, an inherent shutdown assembly typically includes a mass of nuclear absorbant material or poison positioned above the core region and sufficient to shutdown the entire system so as to quickly terminate any dangerous transients. When either the neutron flux or the core cooling temperature exceeds some critical value, a triggered latch is released to permit the poison mass to be inserted into the core. Shutdown assemblies therefore include means to sense either the temperature of the coolant or the activity of the neutron flux, means to hold the poison mass in a position above, and ready to be released into, the reactor core, and means for triggering the release of poison mass into the core when the sensing means indicates that a critical temperature has been reached or the neutron flux has reached a critical value. Present shutdown devices are generally designed for one-time use only. Such designs preclude in situ or ex-reactor testing of the shutdown arrangements. One example of a one-time device is a pelletized poison mass which is restrained by a membrane whose melting point is set at a critical temperature. When a critical temperature is reached the membrane melts, thus releasing the poison into the reactor core. Since it is a one-time-only device, the arrangement cannot be tested for paper operation prior to an actual reactor emergency. Also, present devices have separate sensing, triggering and release elements whose combination and interaction complicates reactor design and encumbers safe operation.
Particular difficulties are encountered in providing ultimate shutdown systems for vertical-type neutronic reactors, particularly existing reactors which are being upgraded or otherwise modified to improve their safety of operation. Vertical reactors have rigid vertical control and safety rods which must, under normal operation, be located outside the reactor core area, and upon demand must be inserted throughout approximately the entire vertical extent of the reactor core. When control rods, safety rods, or the like rigid vertically extending members are employed, the channels or tubes containing such members must extend above the reactor core area, occupying additional space above the reactor, approximately equal to the height of the reactor core. Difficulties are encountered in modifying vertical reactors wherein the control and safety members are positioned to enter a neutronic reactor from above so that in the event of a power failure, the control members will fall in a downward direction under the force of gravity to rest positions within the reactor core, thereby quenching the chain reaction. Such reactor arrangements must include continuous vertical channels or points of access into the reactor core from above. However, it is often difficult to locate such channels, especially in retrofitting a reactor, since some minor horizontal offsets are frequently required to circumvent existing equipment located in the path of the proposed channels. If the control and safety members cannot accommodate a horizontal offset, newly formed vertical access channels must be made to provide communication into the reactor core area. Even if space within and above the reactor is available, such arrangement is undesirable since degradation of containment structure results.
Further, some reactors, such as those used for testing reactor components or the like, have a rotating cover which provides access into the reactor interior. In this arrangement, vertical control rods and the like equipment members extending through the cover must be disconnected to allow rotation of the cap, so as to align passageways in the cap with passageways in the reactor interior. The required provision for disconnection means of shutdown members penetrating into the reactor vessel impairs the reliability of such emergency systems.
It is therefore an object of this invention to provide a compact inherent shutdown assembly for nuclear reactors, particularly vertical-type reactors.
Another object of this invention is to provide an inherent shutdown assembly which is resettable without requiring a dismantling of the reactor core.
Another object of this invention is to provide an inherent shutdown device which, upon demand, extends in a generally downward direction into the reactor core, but which is also capable of negotiating horizontal offsets in its generally vertical movement, to provide a continuous control element extending substantially the entire vertical extent of the reactor core.
Another object of this invention is to provide an inherent shutdown system which is compatable with a wide variety of types of control materials, which is fool-proof in operation, and which is inexpensive to fabricate.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.